Quantifying and Predicting the Presence of Clouds in Exoplanet Atmospheres

One of the most outstanding issues in exoplanet characterization is understanding the prevalence of obscuring clouds and hazes in their atmospheres. The ability to predict the presence of clouds/hazes a priori is an important goal when faced with limited telescope resources and advancements in atmospheric characterization that rely on the detection of spectroscopic features. As a means to identify favorable targets for future studies with HST and JWST, we use published HST/WFC3 transmission spectra to determine the strength of each planet's water feature, as defined by the H2O - J index. By expressing this parameter in units of atmospheric scale height, we provide a means to efficiently compare the size of spectral features over a physically diverse sample of exoplanets. We find the H2O - J index to be strongly correlated with planet temperature when $T_{eq} < 750^{+90}_{-60}$ K and weakly correlated with surface gravity for planets with $\log g < 3.2^{+0.3}_{-0.2}$ dex. Otherwise, the median value of the H2O - J index is $1.8{\pm}0.3$ H. Using these two physical parameters, we identify a division between "classes" of exoplanets, such that objects above $T_{eq} = 700$ K and $\log g = 2.8$ dex are more likely to have clearer atmospheres with stronger spectral features (H2O - J > 1) and those below at least one of these thresholds are increasingly likely to have predominantly cloudy atmospheres with muted spectral features (H2O - J < 1). Additional high-precision measurements are needed to corroborate the reported trends.